Solution-based roll-to-roll deposition techniques are widely considered to be a route to low-cost, high-throughput electronic- and energy-device fabrication. The ink and the deposition method must be carefully designed, however, to produce high-quality semiconductor devices—particularly in the case of solar cells and other optoelectronic semiconductor devices.
For solar cells, for example, it is particularly important that a deposition method be scalable. In addition, the ink should enable dense, thick layers that are without pinholes and result in minimal residual contamination. Further, it is preferable that the solution should be air-stable as well as environmentally friendly.
Several approaches for the fabrication of photovoltaic devices using inorganic nanocrystal inks have been reported in the prior art. Cu2S/CdS heterojunction solar cells having a photovoltaic conversion efficiency of 1.6% have been fabricated using nanoparticles and nanorods. The fabrication of Cu2ZnSnS4 solar cells and large-grained CuInSe2 using sulfide nanocrystal inks has also been demonstrated. Further, nanocrystalline CuInS2 solar cells with approximately 4% efficiency have been fabricated using an in-situ nanocrystal synthesis.
Conventional nanocrystal inks offer advantages over other approaches in that they can be dispersed in organic solvents for coating. Unfortunately, their encapsulating organic ligands can leave behind residues that hurt device performance. Furthermore, nanocrystal synthesis is often air-sensitive, requiring complicated techniques and equipment, such as Schlenk-line manifolds. It has been demonstrated, however, that large-grained Cu(In,Ga)Se2 with efficiencies of >10% can be made using spin-casting precursor inks based on hydrazine. While this is a very exciting approach, hydrazine is toxic and explosive, which makes this approach less desirable.